I placed various objects, plants, toys, etc. upside down and
partially screened, on an overhead projector. I project the
image quite large on the wall. Students draw these as they
enter the room for a bell activity. Sometimes I reveal the
whole form, sometimes not, but it doesn't take long for the
brain to shift into measuring, contour line mode. With
younger students, I use a black collar to hide the objects
from their view so they don't start thinking and drawing
symbols.
(There is the added benefit of a dark, quiet room to help
students settle down to work.)